To prevent unauthorized duplication or alteration of documents, special indicia or background patterns may be provided for sheet materials such as tickets, checks, currency, and the like. The indicia or background pattern is imposed upon the sheet material usually by some type of printing process such as offset printing, lithography, letterpress or other like mechanical systems, by a variety of photographic methods, by xeroprinting, and a host of other methods. The pattern or indicia may be produced with ordinary inks, from special inks which may be magnetic, fluorescent, or the like, from powders which may be baked on, from light sensitive materials such as silver salts or azo dyes, and the like, and from materials that are difficult (e.g., clear varnish) or impossible (e.g., IR and UV emitting materials) to view in the visible light spectrum. Most of these patterns placed on sheet materials depend upon complexity and resolution to avoid ready duplication. Consequently, they add an increment of cost to the sheet material without being fully effective in many instances in providing the desired protection from unauthorized duplication or alteration.
Similar patterns or indicia may be printed on product packaging or directly on products or materials subject to counterfeiting.
Various methods of counterfeit-deterrent strategies have been suggested including Moire-inducing line structures, variable-sized dot patterns, latent images, see-throughs, bar-codes, and diffraction based holograms. None of these methods has proven to be satisfactory.
More successful are methods that involve the use of optically encoded images such as those described in U.S. Pat. Nos. 3,937,565 and 5,708,717 and in U.S. patent application Ser. No. 09/267,420 filed Mar. 11, 1999; U.S. patent application Ser. No. 10/847,943 filed May 18, 2004; U.S. patent application Ser. No. 10/847,962 filed May 18, 2004; U.S. patent application Ser. No. 10/810,000 filed Mar. 26, 2004; and U.S. patent application Ser. No. 11/068,350 filed Feb. 28, 2005, all of which are incorporated herein by reference in their entirety. Optically encoded images typically cannot be discerned or interpreted without a specially tailored optical decoder. They may be used on virtually any form of printed document including legal documents, identification cards and papers, labels, packaging, currency, stamps, etc. The value of using non-reproducible encoded images on documents such as drivers' licenses and vehicle titles is readily apparent. Such images are also highly valuable in their use on packaging as a means of identifying counterfeit goods.
Optically encoded images are typically encoded by one of several methods that involve imposing a regularized periodic pattern on the image or on a background image or pattern. The periodic pattern has a particular predetermined frequency (or frequencies, if multi-dimensional). This may be accomplished through the use of a specialized camera, as described in U.S. Pat. No. 3,937,565, or digitally, as described in U.S. Pat. No. 5,708,717 and the U.S. patent applications referred to above. The image to be encoded can be incorporated into the background pattern or image by introducing distortions to the regular periodic pattern. The high frequency of the regular pattern renders the encoded image difficult or impossible to discern with the naked eye. The image can be readily viewed through the placement of a specially configured refractive decoder lens over the image or, as described in U.S. patent application Ser. No. 11/068,350, through the use of software-based decoding algorithms.
The refractive decoder lenses used to decode encoded images have lenticules or micro-lens elements regularly spaced at a frequency (or frequencies) corresponding to the encoding frequency allows the distortions to be assembled to form the original image. When placed over the encoded image, these lenses allow a viewer to see samples of the image taken at intervals determined by the frequency of the lenticular lens. The lens magnifies these samples and human vision interpolates them into a continuous picture. When oriented at the proper angle, this causes deviations from primary image characteristics having the same frequency to be sampled and magnified, thus standing out from the primary image or background. The action of the lens is essentially to assemble periodic samples of the encoded image into a reconstruction of the original image that was encoded and embedded in the primary image or background.
The typical refractive decoder lens is formed as a transparent or translucent planar element with lens elements (e.g., lenticules or microlenses) formed on one side. The decoder lens must have sufficient thickness to support lens elements with a particular configuration and focal length. Such lenses are typically made from clear plastic and are generally somewhat rigid in order to maintain the relative spacing of the lens elements.
It can be seen from the above that the typical approach to authentication using encoded images is to encode an authentication image and print the encoded image on a document, label or other object to be authenticated. To authenticate the object, the encoded images is positioned for viewing and a refractive decoder lens of the proper frequency is placed over the encoded image in the proper orientation so that the encoded image may be viewed through the lens. The decoded image is then compared to the expected authentication image.
The present invention provides an alternative to the authentication methodology described above and an alternative to the use of refractive lens decoders to decode the encoded images. In the methods of the present invention, the encoded image is printed on a light transmittent (i.e., transparent or translucent) sheet rather than on an opaque surface. The image is decoded by placing the sheet over a reflective decoder surface that has been embossed with a regularized pattern of reflective elements having a frequency (or frequencies) corresponding to the frequency (or frequencies) with which the image was encoded. The embossed pattern may be, for example, a pattern of alternating linear ridges and valleys. The elements may be formed with a particular cross-sectional shape so that light reflected from the embossed pattern is focused in a series of linear samples. The effect is similar to the manner in which a lenticular lens “samples” an image over which it is placed. When the transmittent sheet is placed over the embossed reflective decoder surface in the proper orientation, the light passing through the transmittent sheet is reflected back in a pattern corresponding to the frequency of the embossed pattern. This causes the deviations in the encoded image to stand out from the primary image or background printed on the transmittent sheet. A similar effect analogous to a microarray lens decoder can be achieved through the embossing of a matrix of circular depressions into the reflective surface.